Question: Points $A$, $B$, $C$, and $D$ lie on a line, in that order. If $AB=2$ units, $BC=5$ units and $AD=14$ units, what is the ratio of $AC$ to $BD$? Express your answer as a common fraction.
Explanation: $AC = AB + BC = 7$. $BD = AD - AB = 12$. Thus, $AC:BD=\boxed{\frac{7}{12}}$.